1. Field of the Invention
This invention relates to a system for detecting an abnormal shadow in a radiation image, and more particularly to a system for detecting an abnormal shadow of a breast and/or a bilaterally asymmetric shadow of breasts on the basis of image data representing a radiation image of the breast or breasts.
2. Description of the Related Art
In the medical field, to find a diseased part of a patient or to observe a diseased part of a patient and diagnose progress of disease by reading a radiation image of the object (patient) has been a common operation. However, radiation image reading often depends upon experience and abilities of the reader and is not necessarily objective.
For example, it is necessary to find an abnormal shadow representing a growth and/or a micro calcification representative of a cancerous part in a mammogram (a radiation image of a breast) taken for the purpose of a breast cancer examination. However, depending on the reader, the abnormal shadow cannot be properly detected. Accordingly, there has been a demand to properly detect an abnormal shadow including shadows of a growth and a micro calcification irrespective of abilities of the reader.
In order to meet this demand, there have been proposed abnormal shadow detecting systems (computer-aided image diagnosis systems), for instance, in U.S. Pat. No. 5,761,334 in which an abnormal shadow is automatically detected by the use of a computer on the basis of image data representing a radiation image of the object. In the abnormal shadow detecting system, an abnormal shadow is automatically detected on the basis of the density distribution in the shadow and/or the feature of the shadow by the use of an iris filter processing which is mainly suitable for detecting a growth shadow and/or a morphology filter processing which is mainly suitable for detecting a micro calcification shadow.
The abnormal shadows in the mammogram include a bilaterally asymmetric shadow and an abnormal shadow which appears due to change with time.
The bilaterally asymmetric shadow is a benignant shadow which cannot be perfectly determined not to be a shadow of a cancerous part, belongs to category III (It is said that to detect such an abnormal shadow is important in a medical examination), and is characterized in that the left and right breasts are not symmetric in density.
The abnormal shadow which appears due to change with time is an abnormal shadow representing a part which has been altered into cancer and is characterized in that the mammograms of the same object taken at different times are different from each other in density.
Since ranges of such abnormal shadows are not necessarily circular convex ranges or ranges of high brightness in the mammograms, and are characterized in that a density difference exists between particular images, such abnormal shadows cannot be detected by the conventional iris filter processing or a method simply using the density of the radiation image.
Though various methods of detecting a prospective abnormal shadow related to breast cancer have been proposed, no method of efficiently detecting a bilaterally asymmetric shadow or an abnormal shadow which appears due to change with time has been proposed. Though we, this applicant, have proposed in our U.S. Pat. No. 6,272,233 a method of detecting an abnormal shadow in which the density of a radiation image of the left breast of a patient is compared with that of a radiation image of the right breast of the same patient, the method is for evaluating the probability that a prospective abnormal shadow range detected in one of the radiation image of the left and right breasts represents a true cancer by comparing the output signals obtained by carrying out the iris filter processing on the prospective abnormal shadow range and the corresponding part in the other radiation image, and is not effective for detecting a bilaterally asymmetric shadow.